The disclosed technology relates generally to the formation of transferable micro devices. Semiconductor chip or die-automated assembly equipment typically use vacuum-operated placement heads, such as vacuum grippers or pick-and-place tools, to pick up and apply devices to a substrate. It is often difficult to pick up and place ultra-thin and/or small devices using this technology. Micro transfer printing permits the selection and application of these ultra-thin, fragile, and/or small devices without cause damage to the devices themselves.
Micro-structured stamps can be used to pick up micro devices, transport the micro devices to the destination, and print the micro devices onto a destination substrate. Surface adhesion forces are used to control the selection and printing of these devices onto the destination substrate. This process can be performed massively in parallel. The stamps can be designed to transfer hundreds to thousands of discrete structures in a single pick-up and print operation.
Micro transfer printing also enables parallel assembly of high-performance semiconductor devices onto virtually any substrate material, including glass, plastics, metals or other semiconductors. The substrates can be flexible, thereby permitting the production of flexible electronic devices. Flexible substrates can be integrated in a large number of configurations, including configurations not possible with brittle silicon-based electronic devices. Additionally, plastic substrates, for example, are mechanically rugged and can be used to provide electronic devices that are less susceptible to damage and/or electronic performance degradation caused by mechanical stress. Thus, these materials can be used to fabricate electronic devices by continuous, high-speed printing techniques capable of generating electronic devices over large substrate areas at low cost (e.g., roll to roll manufacturing).
Moreover, these micro transfer printing techniques can be used to print semiconductor devices at temperatures compatible with assembly on plastic polymer substrates. In addition, semiconductor materials can be printed onto large areas of substrates thereby enabling continuous, high-speed printing of complex integrated electrical circuits over large substrate areas. Moreover, fully flexible electronic devices with good electronic performance in flexed or deformed device orientations can be provided to enable a wide range of flexible electronic devices.
Electronically active components can be printed over the non-native substrates. For example, these printing techniques can be used to form imaging devices such as flat-panel liquid crystal, LED, or OLED display devices and/or in digital radiographic plates. In each instance, the electronically active components must be transferred from a native substrate to a destination substrate (e.g., a non-native substrate on which an array of the active components is distributed). The active components are picked up from the native substrate and transferred to the destination substrate using an elastomer stamp. The release of the active components must be controlled and predictable.
In micro-transfer printing, the chiplets are typically formed on a silicon substrate and a sacrificial layer undercut by etching to form tethers using photolithographic processes. The silicon substrate facilitates the formation of tethers between the wafer and the chiplet that are broken to release the chiplet during the micro-transfer printing process. However, conventional methods of forming GaN devices do not result in formation of micro-scale devices that can be assembled using micro-transfer printing techniques. Additionally, conventional methods of forming GaN devices do not enable printable GaN devices on sapphire. Although relatively inexpensive when compared to sapphire, silicon has an even larger lattice mismatch with the GaN crystal structures making up the LEDs than sapphire, further reducing the performance of the resulting LEDs. Thus, in some embodiments, it is desirable to form printable structures, such as LEDs, using a sapphire substrate. However, there is no available method for undercutting a chiplet formed on a sapphire substrate to enable chiplet release for micro-transfer printing.
There is a need, therefore, for structures and methods that enable the construction of micro-LED chiplets formed on a substrate (e.g., silicon or sapphire) that can be micro-transfer printed. There is also a need for simple and inexpensive methods and structures enabling electrical interconnections for chiplets printed on destination substrates. Furthermore, there is a need for methods and structures that allow electrically connecting the electrical contacts of printed structures, such as printed LEDs, using fewer processing steps than conventional methods.
Thus, there is a need for predictable and controllable systems and methods for preparing high-performance, small, and dense arrays of structures in GaN and related materials that are suitable for micro-transfer printing.